DMs are not retrieved until the application is restarted

[dtonon]

After a while that Gossip is open, DMs (I'm noticed this with NIP-17 ones) are not retrieved even if the relay where they are stored is green and last connection shows a recent activity; if the application is restarted, they immediately flush in.

Running Unstable 524d3a69

[mikedilger]

Probably the long-lived subscription that loads them was somehow closed. We should detect this, pause, and then restart any long-lived subscription that drops.